Method of preparing low-loss insulating material



Patented May 30, 1944 METHOD OF PREPARING LOW-LOSS INSULATING MATERIAL iii-nest L. Dinning, Jr., Baltimore County, Md.

No Drawing. Application May 29, 1942,

Serial No. 445,100

17 Claims.

This invention relates to the preparation of insulating materials having low electrical losses and more particularly to the preparation of material of this nature from a drated magnesium silicate in which a considerable portion of a magnetic metal is present as an impurity in the form of one or more chemical compounds.

In the manufacture of ceramic insulation having low power loss when situated in a high frequency alternating electrical field, it has been the custom to pulverize one of the many types of steatite found in nature or to use a steatite occurring naturally in the powdered form, mould the powder with a suitable binding agent into the desired form and then fire the moulded piece at an elevated temperature, hardening the moulded piece in the required form with the desired dimensions. Considerable shrinkage takes place during the firing operation and it is impossible, using this process, to manufacture parts to accurate, uniform dimensions withput a prohibitive percentage of rejection. In parts where dimensional accuracy is extremely important, natural steatitic rock, taken from the earth, is machined to the desired form and then fired to harden it, as there is little shrinkage in this substance when it is kilned. The kilned rock is employed only where absolutely necessary as its electrical losses exceed those of the fired moulded powder and because rock of suitable composition for even' the existing high loss material is found at only a few places in the world. It has been necessary in both of the above processes to use only steatites substantially free of impurities comprising compounds of the magnetic group of metals in the previous manufacture of insulation having low high frequency losses. As a result, the number of natural deposits of steatite whose output may be utilized in the manufacture of insulation of this class has been quite circumscribed, rendering useless to the industry the major portion of known deposits.

One of the principal objects of this invention is to provide a new and novel process for the preparation of low loss electrical insulation from steatite compositions hearing as an impurity an appreciable percentage of a magnetic metal or,

compound thereof.

Still another object of the invention is to produce a high quality electrical insulation material from widespread steatitic deposits whose output was formerly unusable.

Yet another object of the invention is to proelectrical insulation from a steatite composition bearing several percent of iron. I

A further object of the invention is to provide a process for the production of low loss electrical insulators with very close dimensional tolerances.

The designation steatite as employed in the specification and claims is intended to broadly cover the forms in which magnesium silicate, aluminum silicate and magnesium aluminum silicate occurs in nature, including soapstdie, talc, and the material designated by the trade name Lava. Likewise, magnetic impurities" as employed in the specification and claims is intended to comprise any magnetic metal or chemical compound thereof.

In the preparation of low loss insulation from steatitic materials containing several percent of magnetic impurities according to the method of our, invention, a block of steatite as taken from the deposit is machined to the required dimensions by any of the well known processing means; such as sawing, milling, drilling, etc. In its unfired, or native, state, the steatite piece is termed green," to contrast it with a fired piece. The

machined product is then hardened by firing it at a temperature of 1500 F. to 2300 F. in a carbonaceous reducing atmosphere obtained in any of several ways. Ways which have proven suitable for the production of such an. atmosphere are; the introduction of a gaseous hydrocarbon or an incompletely oxidized gaseous carbon compound, placing the object within a graphite or carbon crucible, or placing it on a carbon containing supporting means such as a graphite sagger. I have found that forms containing iron as an impurity, which have been treated in the aoove manner, are white or gray in color and have excellent electrical properties as compared to the brown coloration and poor electrical properties obtained when the same material is fired without making the above modifications in the firing atmosphere.

In the preceding paragraph outlining the steps of the process, reference is made to a carbonaceous reducing atmosphere. This term is intended to define any carbon containing compound gaseous at the firing temperature which has the property of removing oxygen from the oxygen compounds of the magnetic impurity present in the form being fired at the kilning temperature. In experiments on a form containing iron as an impurity, it was found that the material turned adark brown and had very'high electrical losses when the firing operation was performed in the vide a process for the preparation of low loss 5 Presence of y n- The 01 1 n h h 10 888 the iron in an oxide intermediate the losses were obtained and the form was found to be a white or gray-white in color. It is not known precisely, but the low losses and color of the form are here'attributed to the presence of ferrous and ferric forms.

Using material from a certain deposit having the approximate composition after firing $102- 62%, MgO-28.25%, Fe-'7%, Alma-2.5%, and S-0.25% the following results were obtained at 1000 kilocycles:

- Fired on Fired on v graphite midmary .sagger re rectory sagger Dielectric constant 5. 65 6. 2 Power factor 0. 35 l. Loss factor l. 98 6. 2 Water absorption of waxed sample 0. 007 0. 014

As is observed, more than a three-to-one improvement in overall electrical properties is realized by the use of the above process and at the same time water absorption is halved. On firing the same material at 1800" F. to 2000 F. in an atmosphere of hydrogen in accordance with the method now in use on the special materials mentioned earlier, the forms emerged from the firing permeated with a black coloration and having very inferior electrical properties. The only steatite or lava suitable in composition for hydrogen firing is of foreign origin and must be substantially free of magnetic impurities. During times of international conflict, it is often obtainable only with difiiculty or not at all. The method which I have described permits the production of high quality low loss electrical insulation material from domestic deposits of steatite or lava containing magnetic elements as impurities, and which have heretofore been unsuitable for use. The quality of the resultant product is substantially equal to that hitherto obtained only by the hydrogen firing of special imported lava.

The method of the invention may be applied with equal benefit to insulating bodies machined out of the native stcatite blocks and to those manufactured by the moulding of powdered steatite with a suitable binder to the desired shape. In connection with the latter, it may be mentioned that the shrinkage resulting from the firing operation has been found markedly smaller than that occurring in most of the now widely used moulding substances of this nature. It is possible that this may be due to the presence of the ma netic impurities which, up to the present time, have rendered such materials unsatisfactory for the manufacture of low loss insulation.

It has been found that the reaction which occurs in iron bearing substances during the firing operation is reversible. Thus, the material may be heated in air to a temperature in excess of 1500 F. to harden it after the proper shaping of the green form. The insulator is now brown in color and has relatively poor electrical qualities. Upon reheating the insulator to a temperature in excess of 1550" F. in contact with a carbona-- ceous reducing atmosphere, the color changes to gray or grayish-white and the electrical properties improve and become equivalent to those of the forms which are originally fired in contact with the necessary reducing atmosphere. Reheating the insulator in the presence of oxygen brings about reversion to the brown form with poor electrical properties, which may be changed to the gray form by repeating the second treatment. The above cycle may be repeated as often as desired.

It will be obvious that many changes and modifications may be made in the invention without departing from the spirit thereof as expressed in the foregoing description and in the appended claims.

Iclaim:

1. The method of producing low loss electrical insulation which comprises firing a body of green steatitic material containing more than two percent by weight of iron at a temperature in excess of 1500" F. in a carbon containing crucible and substantially excluding air and hydrogen from said body during said firing and subsequent cooling of said body.

2. The method of producing a low loss electrical insulator which comprises firing a body of green steatitic material containing more than two percent by weight of iron at a temperature in excess of 1500 F. supporting said body on carbon containing supporting means during the firing operation and substantially excluding air and hydrogen from said body during said firing and subsequent cooling of said body.

3. The method of producing a low loss electrical insulator which comprises firing a body of steatitic material containing more than two percent by weight of iron and more than one-tenth percent by weight of sulphur at a temperature in excess of 1500 F. in the presence of' a reducing atmosphere having a lower limit of activity sulficient to prevent said body from turning brown during the firing operation and having an upper limit of activity insufilcient to cause blackening of said body.

4. The method of producing low loss electrical insulation which comprises heating a body of steatitic material containing oxides of magnetic elements in a hydrogen free carbonaceous reducing atmosphere to a temperature at which said oxides are partially reduced.

5. The method of producing low loss electrical insulation from steatitic material containing an oxide of iron which comprises, heating a body of said material in a hydrogen free carbonaceous reducing atmosphere to a temperature at which said oxide is partially reduced,

6. The method of producing low loss electrical insulation which comprises heating a body of steatitic material containing ferric oxide in a hydrogen free carbonaceous reducing atmosphere to a temperature at which said oxide is partially reduced.

'7. The method of producing low loss electrical insulation which comprises heating a body of steatitic material to a temperature in excess of 1500 F. in a free oxygen bearing atmosphere, and subsequently subjecting said body to the action of a carbonaceous reducing atmosphere at a temperature exceeding 1550 F.

8. The method of producing low loss electrical insulation which comprises heating a body of steatitic material containing more than twd percent by weight of magnetic impurities in contact ing composition comprising steatitic material con-' taining magnetic metal impurities fired at a temperature in excess of 1500 F. in the presence of carbon in an atmosphere substantially free from oxygen. Y

10. A low loss high frequency electrica1.insu-' lating composition comprising steatitic material containing. magnetic metal impurities fired at a temperature greater than 1500 F. in a carbon containing crucible irom which air is substantially excluded.

11. A low loss highJrequency electrical insulating composition comprising steatitic material containing magnetic metal impurities fired at a temperature in excess of 1500 F. while supported on carbon containing supporting means in an at mosphere substantially free from oxygen.

12. A low loss high frequency electrical insulating composition comprising steatitic material containing more than two percent by weight of magnetic metal impurities fired at a temperature in excess of 1500 F. in the presence of carbon in an atmosphere substantially free from oxygen.

13. A low loss high frequency electrical insu lating composition comprising steatitic material containing more than two percent by weight of magnetic metal impurities fired at a temperature in excess of 1500" F. in a carbon containin crucible from which air is substantially excluded.

I 14. A low loss high frequency electrical insulating composition comprising steatitic material containing more than two percent .by weight of magnetic metal impurities fired at a temperature in excess of 1500 F. while supported on carbon 10 containing supporting means in an atmosphere substantially free from oxygen.

15. A low loss high frequency. electrical insulating composition comprising steatitic material containing more than two percent by weight of 15 iron fired at a temperature in excess of 1500? I".

in the presence of carbon in an atmosphere substantially free from oxygen.

16. A low loss high frequency electrical insulating composition comprising steatitic material 20 containing more than two percent by weight of iron fired at a temperature in excess of 1500" F. in a carbon containingcrucible from which air is substantially excluded.

1'1. A low loss high frequency electrical insulating composition comprising steatitic material containing more than two percent by weight of iron fired at a temperature in excess of 1500 F. while supported on carbon containing supporting means in an atmosphere substantially free from 30 oxygen.

ERNEST L. DINNING, J R. 

